The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to the rapid acquisition of three-dimensional MR images.
Any nucleus that possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant γ of the nucleus). Nuclei which exhibit this phenomena are referred to herein as “spins”.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment Mz is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance (“NMR”) phenomena is exploited.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (Gx, Gy, and Gz) which have the same direction as the polarizing field B0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified. One such method is disclosed in U.S. Pat. No. 5,532,595 which is incorporated herein by reference. This so-called “shell” k-space sampling trajectory samples a spiral pattern around a spherical surface. A complete image acquisition is comprised of a series of such spiral sampling patterns over a corresponding series of spheres of increasing diameter.
Object motion during the acquisition of NMR image data produces both blurring and “ghosts”. Ghosts are particularly apparent when the motion is periodic, or nearly so. For most physiological motion each view of the NMR signal is acquired in a period short enough that the object may be considered stationary during the acquisition window. In such case the blurring and ghosting is due to the inconsistent appearance of the object from view to view. Motion that changes the appearance between views such as that produced by a patient moving, by the respiration or the cardiac cycle, or by peristalsis, is referred to hereinafter as “view-to-view motion”. Motion may also change the amplitude and phase of the NMR signal as it evolves during the pulse sequence and such motion is referred to hereinafter as “in-view motion”.
Both blurring and ghosting can be reduced if the data acquisition is synchronized with the functional cycle of the subject to reduce view-to-view motion. This method is known as gated or triggered NMR scanning, and its objective is to acquire NMR data at the same point during successive functional cycles so that the subject “looks” the same in each view. The drawback of gating is that NMR data may be acquired only during a small fraction of the subject's functional cycle, and even when the shortest acceptable pulse sequence is employed, the gating technique can significantly lengthen the data acquisition.
Another method for reducing motion artifacts is to correct the acquired data to offset the effects of patient motion. As described for example in U.S. Pat. Nos. 4,937,526 and 5,539,312, this requires that so-called “navigator signals” be periodically acquired during the scan. The navigator signal data are used to retrospectively correct the acquired NMR signals for patient motion or to prospectively correct for patient motion by altering the pulse sequence. Such navigator signal acquisitions are in addition to the NMR data acquisitions, and they therefore can significantly lengthen the total scan time.
In conventional, fully-sampled MRI, the number of acquired k-space data points is determined by the spatial resolution requirements, and the Nyquist criterion for the alias-free field of view (FOV). Images can be reconstructed, however, using a reduced number of k-space samples, or “undersampling”. The term undersampling here indicates the Nyquist criterion is not satisfied, at least in some regions of k-space. Undersampling is used for several reasons, including reduction of acquisition time, reduction of motion artifacts, achieving higher spatial or temporal resolution, and reducing the tradeoff between spatial resolution and temporal resolution. Aliasing artifacts that result from undersampling are not as severe if the violation of the Nyquist criterion is restricted to the outer part of k-space.
The time required to fully sample 3D Cartesian k-space is relatively long. Alternative non-Cartesian trajectories can provide faster coverage of k-space, and more efficient use of the gradients. When a very fast volume acquisition is required, undersampling strategies can be used in conjunction with these non-Cartesian trajectories to further reduce the scan time. The method of Lee J H, Hargreaves B A, Hu B S, Nishimura D G; Fast 3D Imaging Using Variable-Density Spiral Trajectories With Applications To Limb Perfusion, Magn. Reson. Med. 2003; 50(6): 1276-1285, uses a variable-density stack of spiral trajectories that varies the sampling density in both the kx-ky plane and the kz direction. That method preserves reasonable image quality, while reducing the acquisition time by approximately half compared to a fully-sampled acquisition. Vastly undersampled 3D projection acquisition as described by Barger V A, Block W F, Toropov Y, Gist T M, Mistretta C A, Time-Resolved Contrast-Enhanced Imaging With Isotropic Resolution and Broad Coverage Using An Undersampled 3D Projection Trajectory, Magn. Reson. Med. 2002; 48(2):297-305, has been used to increase temporal resolution and provide better dynamic information for 3D contrast-enhanced MRA. The aliasing caused by undersampling in this method can be tolerated because the vessel-tissue contrast is high and the artifacts are distributed, or spread out in the image.